Patients are often treated for diseases and/or conditions associated with a compromised status of the patient, for example a compromised physiologic status. Such conditions may include sleep apnea, which is implicated in atrial fibrillation, hypertension, and chronic fatigue; heart failure, asthma, chronic obstructive pulmonary disease and others. In some instances, a patient may report symptoms that require diagnosis to determine the underlying cause. In such cases, long term monitoring of the patient can provide useful information as to the physiologic status of the patient. In some instances a patient may have been hospitalized and monitoring is necessary in the intensive care unit or post-anesthesia. One example of a device to provide long term monitoring of a patient is the Holter monitor, or ambulatory electrocardiography device, which may use electrodes attached to the skin to measure electrocardiogram signals from the patient.
While effective, Holter monitors are bulky, uncomfortable and do not lend themselves to every application. For example, athletes and other non-patients may want to monitor respiration and/or breath volume for training and conditioning purposes or during competition. Conventional devices, such as the Holter monitor, may not collect all of the kinds of data that would be ideal to diagnose respiration rate or the tidal volume thus making it ineffective for diagnosing and/or treating a patient for apnea. In addition, because a Holter monitor is uncomfortable and bulky, it may result in a “non-compliant” patient or individual because the individual refuses to wear the device and any data that is collected may be incomplete and less than ideal.
Other conventional devices measure physiologic parameters typically by impedance measurements. For example, transthoracic impedance measurements can be used to measure hydration and respiration. Although transthoracic measurements can be useful, such measurements may use electrodes that may be somewhat uncomfortable and/or cumbersome for the individual to wear making long term monitoring more difficult.
Respiratory Inductance Plethysmography (RIP) is a method of evaluating pulmonary ventilation by measuring the movement of the chest and abdominal wall. Accurate measurement of pulmonary ventilation or breathing often requires the use of devices such as masks or mouthpieces coupled to the airway opening. These devices are often both encumbering and invasive, and thus ill-suited for continuous or ambulatory measurements. As an alternative, RIP devices that sense respiratory excursions at the body surface can be used to measure pulmonary ventilation, but are difficult to calibrate and normally only used to measure “effort” not volume. Further, an RIP measurement is reset on every cycle, so no indication of longer term girth size change, drift and/or migration is recorded.
Thus, several sensor methodologies based on this theory have been developed using single elastic bands and dual elastic bands. The elastic transducer bands typically include an embroidered two braided sinusoid wire coils that are insulated by fabric in a lightweight elastic and adhesive band. The transducer bands are placed around the rib cage under the armpits and around the abdomen at the level of the umbilicus (belly button). They are connected to an oscillator and subsequent frequency demodulation electronics to obtain digital waveforms. During inspiration the cross-sectional area of the rib cage and abdomen increases altering the self-inductance of the coils and the frequency of their oscillation, with the increase in cross-sectional area proportional to lung volumes. The electronics convert this change in frequency to a digital respiration waveform where the amplitude of the waveform is proportional to the inspired breath volume. However, this methodology requires expensive electronics and essentially passes radio waves through the body which may prove deleterious to an individual's health.
RIP technology has been incorporated into stretch garments and bands but the state of the art in resistive stretch sensors is not ideal. Typically the resistance increases for a certain percentage of stretch up to a point then decreases with increasing stretch. In other words, there is not a good 1:1 correlation between stretch length and resistance. Further the resistance is temporarily affected by the change in length and the speed of that change resulting in situations in which the resistance actually increases when the material recovers to shorter lengths after being stretched. Moreover, knitted, stretch fabrics are extremely variable as are the electrical characteristics of knitted fabrics with conductive/resistive threads. Further, motion artifact affects accurate measurements with changes in resistance tied to motion and length.
Thus, known methods and devices for long term monitoring of individuals may be less than ideal. At least some of the known devices may not collect the right kinds of data to treat patients optimally and are not readily available to athletes and other individuals who want to use the devices for non-medical reasons such as for training purposes.
Therefore, a need exists for an improved, comfortable, continuous, ambulatory monitoring system and method that is capable of providing accurate respiratory volumetric dynamics data and that overcomes the short-comings of conventional methods and devices.